1. Field of the Invention
This invention relates to a small-size solid electrolytic capacitor using a high melting point material such as tantalum or niobium as a valve-acting metal and a method of manufacturing such a solid electrolytic capacitor.
2. Description of the Related Art
Solid electrolytic capacitors using a high melting point material such as tantalum or niobium as a valve-acting metal for an anode are small in size, large in capacitance, and excellent in frequency characteristics and have thus been widely used in decoupling circuits of CPUs (Central Processing Units) installed in electronic devices, power circuits of electronic devices, and so on. Such solid electrolytic capacitors are also installed in many portable electronic devices and, following the development of portable electronic devices, have been required to be still smaller in size and still larger in capacitance. Particularly, some of the small-type capacitors are called chip-type solid electrolytic capacitors each in the form of a product generally having an external shape of a rectangular parallelepiped with sides of several millimeters or less and covered with a casing resin.
The structure of a conventional general chip-type solid electrolytic capacitor will be described with reference to FIG. 4. FIG. 4 is a see-through diagram, as seen in a side direction, of the structure of the conventional chip-type solid electrolytic capacitor. In FIG. 4, an anode lead 1 is made of a metal such as tantalum or niobium, a porous sintered body 2 of the same kind of metal as the anode lead 1 is formed around the anode lead 1, and part of the anode lead 1 protrudes from the porous sintered body 2. Further, an anodized film layer is formed by anodic oxidation (anodization) at the surface of the porous sintered body 2. A solid electrolyte layer is formed on the surface of the anodized film layer and, on the surface of the solid electrolyte layer, a graphite layer and a cathode layer are formed in the order named. The anode lead 1, the porous sintered body 2, the anodized film layer, the solid electrolyte layer, the graphite layer, and the cathode layer are collectively called a capacitor element. In FIG. 4, however, the anodized film layer, the solid electrolyte layer, the graphite layer, and the cathode layer formed outside the porous sintered body 2 are not illustrated because they are thin as compared with the porous sintered body 2.
The fabrication sequence of this capacitor element will be described in the case of the capacitor being a tantalum solid electrolytic capacitor. At first, there are prepared a large number of rod-like anode leads 1 made of tantalum serving as a valve-acting metal. Then, each anode lead 1 is buried in porous tantalum powder, which is then subjected to press molding, thereby obtaining a porous body with one end of the anode lead 1 protruding from the porous body. Then, the porous body with the anode lead 1 is subjected to vacuum sintering, thereby obtaining a porous sintered body 2 with the anode lead 1. Then, a metal frame made of, for example, a stainless steel is prepared and end portions of the anode leads 1 protruding from the porous sintered bodies 2 respectively, are attached to the metal frame by welding or the like. The porous sintered bodies 2 attached to the metal frame through the anode leads 1 are immersed in an anodizing solution such as a hot phosphoric acid aqueous solution and a voltage is applied thereto to perform anodic oxidation to anodize the surfaces of the porous sintered bodies 2, thereby forming an anodized film layer at the surface of each porous sintered body 2. Then, a solid electrolyte layer is formed on the surface of each anodized film layer and, further, a graphite layer and a cathode layer are formed in the order named on each solid electrolyte layer at least at a part thereof, thereby obtaining capacitor elements.
Then, the end portions of the anode leads 1 fixed to the metal frame are cut so as to separate the capacitor elements from the metal frame. Since the length of the protruding portion of each anode lead 1 is shortened by this separation, it is necessary that the protruding length be determined taking into account a cutting margin in advance. Further, if droplets adhere to the metal frame, with the anode leads welded thereto, by evaporation and reliquefaction of the anodizing solution during the anodic oxidation, a bridge is formed between the metal frame and the surface of the anodizing solution due to the adhesion of droplets. In this case, both are shorted together so that it is not possible to apply an anodization voltage necessary for forming an anodized film layer at the surface of each porous sintered body 2. Therefore, it is necessary that the protruding length of each anode lead 1 be set to a value that allows the metal frame to be away from the surface of the anodizing solution so as to prevent occurrence of a short circuit. Since an anodized film layer is formed during the anodic oxidation on the protruding portion of each anode lead 1, part of it is stripped if necessary after the separation of the metal frame. Then, the anode lead 1 and the cathode layer of each capacitor element are electrically connected to two electrode terminals, respectively. Thereafter, each capacitor element is coated with an insulating casing resin 9 such that the electrode terminals are partly exposed on the surface of the resin, thereby obtaining chip-type solid electrolytic capacitors.
In the conventional example shown in FIG. 4, the anode lead 1 is connected to a mounting anode terminal member 5 through a metal bonding member 14 and the cathode layer is connected to a mounting cathode terminal member 4 through a conductive adhesive 7. The bonding member 14 may be made of any conductive material and is generally in the form of a metal block such as an aluminum block. Generally, the anode lead 1 and the bonding member 14 are fixedly connected together by resistance welding and the bonding member 14 and the mounting anode terminal member 5 are also fixedly connected together by resistance welding. The bonding member 14 has a trapezoidal shape in FIG. 4, but may have any shape depending on the necessity of design of a chip-type solid electrolytic capacitor. In FIG. 4, the anode lead 1 is welded to a side surface of the bonding member 14 and the bonding member 14 is disposed behind the anode lead 1 with respect to the sheet surface. On the other hand, the anode lead 1 is embedded so as to substantially pass through the middle of the porous sintered body 2.
Japanese Unexamined Patent Application Publication (JP-A) No. 2001-244147 (Patent Document 1) and Japanese Unexamined Patent Application Publication (JP-A) No. 2001-267181 (Patent Document 2) describe chip-type solid electrolytic capacitors different from the conventional structure described above. The chip-type solid electrolytic capacitor described in Patent Document 1 is characterized by using a metal wire with a fuse function, instead of the metal block, as the bonding member used in the foregoing conventional example, wherein use is made, as the metal wire, of a copper alloy or a 42 alloy (an alloy containing 42 wt % Ni and 58 wt % Fe) having a relatively low melting point. A mounting anode terminal member is provided at the bottom of the chip-type solid electrolytic capacitor so as to be partly exposed from an insulating casing resin and the metal wire is connected to the mounting anode terminal member by wire bonding or the like. On the other hand, the metal wire and an anode lead are bonded together under heat and pressure.
Also in Patent Document 2, an anode lead and a mounting anode terminal member are connected together by a metal wire. Like in Patent Document 1, a copper alloy or a 42 alloy is used as the metal wire. What is called the metal wire in Patent Document 2 is a columnar metal piece with a square or circular cross-section and with a size similar to that of the bonding member in the form of the metal block shown in FIG. 4. The metal wire and the anode lead are electrically connected together by resistance welding, while, the metal wire and the mounting anode terminal member are electrically connected together by a conductive adhesive.